Your shots of musicians are great. You are able to capture the emotion in their performances.
He appears to be playing a four string guitar. Maybe some of the other guitar players can correct me, but I think a four string guitar is tuned in fourths like a violin rather than fifths like a six string guitar.
A 'four string guitar' is usually a baritone ukulele. Same tuning as the top four strings of a normal guitar. This is 'fourths' (more or less).
A violin (and mandolin) is tuned in 'perfect fifths'.
With ukulele tuning, guitar chords are used ignoring the missing two strings in the 5th and 6th positions. If violin/mandolin tuning is used, it can still be thought of in terms of guitar chords but they are reversed and the two former top string positions are missing and it's in a different key. It's much easier to do it than explain it!
It's difficult to know which tuning he is using - he may be using something else of his own invention.
If it was my instrument, I would use either mandolin tuning (but probably transposed down five semitones) or open G banjo tuning.
I bet it's a mandolin of some sort. The fourth string is thicker than the third string, so it doesn't have a high fourth string like in traditional uke tunings, though of course lots of modern players use a low bottom string for more range, so that's not definitive. It's got an archtop style bridge like a mando, though of course there are archtop guitars, so that's not definitive either. He's using two contact pickups like the mando players seem to like, which isn't definitive either, but points in the general mando direction.